Proxy Design Pattern- tutorialspoint.com example

问题:

http://www.tutorialspoint.com/design_pattern/proxy_pattern.htm

Hi,

I am looking to understand proxy design pattern in java using the example in the link above. In main method I don't understand the difference between:

  //image will be loaded from disk

  image.display(); 
  System.out.println("");

  //image will not be loaded from disk

  image.display();  

Is this a typo? How can the same 2 image.display() methods give different outputs?

Thanks a lot!

回答1:

This is not a typo. If you look in the definition of ProxyImage in the tutorial, it clearly has state:

public class ProxyImage implements Image{

   private RealImage realImage;
   private String fileName;

   public ProxyImage(String fileName){
      this.fileName = fileName;
   }

   @Override
   public void display() {
      if(realImage == null){
         realImage = new RealImage(fileName);
      }
      realImage.display();
   }
}

When the first call is made, realImage is null, and the image will be loaded from disk. After that, the loaded image is stored to image.realImage and is displayed. At the second call that image is already cached, and hence no load from disk is necessary.

回答2:

No, it's not a typo.

• The first time you call image.display(), it loads the image from disk. It stores what it loaded, so...
• The second (and subsequent times) you call it, it doesn't need to load the image from disk again.

回答3:

The only difference and easy way to understand it is by using constructor and calling the functionality method, which is to load the image here. If you instantiate your interface with the implementation of this non proxy MainFunctionalityClass and that implementation loads the image inside this constructor. However, using proxy is to apply a wrapper over the MainFunctionalityClass and this time, the called constructor is that of the Proxy's, it means, the MainFunctionalityClass constructor is skipped here.

    class MainFunctionalityClass implements ICommon{

    MainFunctionalityClass(){
     loadMainFunctionality(); // Costly call
    } 
}

While the proxy class is defined as:

class Proxy implements ICommon{
 private String var;

Proxy(String var){
this.var = var; //NO call to big functionality, not costly 
}

callMainFunctionality(){
//uses var, MainFunctionalityClass() called
// Costly call but it is called on demand, 
}

class MainClassTest{
 public static void main(String[] args){
 ICommon p = new Proxy("abcd");
p.callMainFunctionality();
}

}

Proxy Design Pattern can be used for

1. Loading big functionality on demand lazily.
2. Restricting the operations of the client(not explained above)

To implement it we need to first to use same interface in both the classes and then to use composition in the proxy class.